Low power free running oscillator

ABSTRACT

Various embodiments relate to a free running oscillator, that includes a switch capacitor based frequency-to-voltage converter (F2V), a comparator, and a voltage controlled oscillator (VCO), which may be collectively configured to reduce amplifier offset and flicker noise while increasing effective gain of the amplifier of the comparator. The F2V may produce a feedback voltage Vfb corresponding to frequencies of output of the VCO. The comparator may be configured to sample a reference voltage Vref using a sampling capacitor, compare Vref to Vfb, and generate an output based on any difference between Vref and Vfb, where the output may be integrated using an integrating capacitor of the comparator. The comparator may compensate for parasitic capacitance at the output of the amplifier by using an amplifier having two outputs, with the sampling capacitor and integrating capacitor being coupled to respectively different outputs of the amplifier.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional claiming benefit of the filing date ofprior U.S. patent application Ser. No. 17/362,353, filed Jun. 29, 2021,the contents of which are incorporated by reference herein.

TECHNICAL FIELD

Various exemplary embodiments disclosed herein relate generally to lowpower free running oscillator (FRO) combining low phase noise with highfrequency stability.

BACKGROUND

Low power silicon-based FROs with high frequency accuracy and low phasenoise are indispensable for many products. Especially for low powerwireless systems (IoT, BLE, key fobs) energy is often scarce because itis delivered by batteries or even by energy harvesting.

SUMMARY

A summary of various exemplary embodiments is presented below. Somesimplifications and omissions may be made in the following summary,which is intended to highlight and introduce some aspects of the variousexemplary embodiments, but not to limit the scope of the invention.Detailed descriptions of an exemplary embodiment adequate to allow thoseof ordinary skill in the art to make and use the inventive concepts willfollow in later sections.

Various embodiments relate to a free running oscillator, including: avoltage controlled oscillator circuit including an input configured toreceive an input voltage and an output configured to provide anoscillation signal, wherein the input voltage controls a frequency ofthe oscillation signal; a frequency to voltage circuit including aninput configured to receive the oscillation signal and an outputconfigured to produce a voltage dependent on a frequency of theoscillation signal; a comparison circuit including an input and anoutput including: a first amplifier including a first input, a secondinput, and an output, wherein the output is based upon a difference involtage between the first input and the second input, wherein the firstinput received one of a reference voltage and the output of frequency tovoltage circuit; a second amplifier including a first input, a secondinput, and an output, wherein the output is based upon a difference involtage between the first input and the second input, first input isconnected to the comparator output, the second inputs is connected tothe second amplifier output; a sampling capacitor connected between thesecond input of the first amplifier and a ground; and an integrationcapacitor connected between the comparator output and the ground.

Various embodiments are described, wherein a first switch with an outputconnected to the first input of the amplifier and a first inputconnected to the reference voltage and a second input connected to theoutput of frequency to voltage circuit, wherein the first switchswitches between the first input and the second input to produce anoutput signal at the first switch output; a second switch connectedbetween the amplifier output and the comparator output; a third switchconnected between the output of the first amplifier and the second inputof the first amplifier; and a fourth switch connected between the outputof the second amplifier and the output of the first amplifier.

Various embodiments are described, wherein the voltage controlledoscillator circuit produces a first phase signal, a second phase signal,and a third phase signal, the first phase signal controls the firstswitch and the third switch between a sampling phase and comparisonphase, the second phase signal controls the fourth switch during thecomparison phase to charge a parasitic capacitance at the output of thefirst amplifier, and the third phase signal controls second switchduring the comparison phase to connect the output of the first amplifierto the output of the comparator.

Various embodiments are described, wherein second phase signal activatesthe fourth switch during a timeframe different from the timeframe whenthe third phase signal controls the second switch.

Various embodiments are described, wherein the sampling phase includessampling the reference voltage and storing the sampled reference voltageon the sampling capacitor.

Various embodiments are described, wherein the comparison phase includescomparing a voltage on the sampling capacitor with the output of thefrequency to voltage circuit.

Various embodiments are described, wherein the first amplifier is anoperational amplifier wherein the output is a current based upon thedifference in voltage between the first input and second input of thefirst amplifier.

Various embodiments are described, wherein the second amplifier is anoperational amplifier wherein the output is a current based upon thedifference in voltage between the first input and second input of thesecond amplifier.

Further various embodiments relate to a free running oscillator,including: a voltage controlled oscillator circuit including an inputconfigured to receive an input voltage and an output configured toprovide an oscillation signal, wherein the input voltage controls afrequency of the oscillation signal; a frequency to voltage circuitincluding an input configured to receive the oscillation signal and anoutput configured to produce a voltage dependent on a frequency of theoscillation signal; a comparison circuit including an input and anoutput including: an amplifier including a first input, a second input,a first output, a second output, and a first switch, wherein the firstand second output is based upon a difference in voltage between thefirst input and the second input, the first switch activates one of thefirst output and the second output, and the second output is connectedto the second input; a second switch with an output connected to thefirst input of the amplifier and a first input connected to a referencevoltage and a second input connected to the output of frequency tovoltage circuit, wherein the second switch switches between the firstinput and the second input to produce an output signal at the secondswitch output; a sampling capacitor connected between the second inputof the first amplifier and a ground; and an integration capacitorconnected between the first comparator output and the ground.

Various embodiments are described, wherein the voltage controlledoscillator circuit produces a control signal configured to control thefirst switch and second switch.

Various embodiments are described, wherein a sampling phase includessampling the reference voltage and storing the sampled reference voltageon the sampling capacitor based upon the control signal.

Various embodiments are described, wherein a comparison phase includescomparing a voltage on the sampling capacitor with the output of thefrequency to voltage circuit based upon the control signal.

Various embodiments are described, wherein the amplifier is anoperational amplifier wherein the output is a current based upon thedifference in voltage between the first input and second input of thefirst amplifier.

Further various embodiments relate to a method for operating a freerunning oscillator system, the method including operating the oscillatorsystem during a plurality of phases of a comparison circuit of theoscillator system occurring periodically, including: during a firstphase of the plurality of phases, providing a reference voltage to afirst input of a first amplifier via a first switch, wherein a secondinput of the first amplifier is coupled via a first switch to an outputof the first amplifier and is coupled to a sampling capacitor, wherein avoltage of the output of the first amplifier is provided to the samplingcapacitor; during a second phase and a third phase of the plurality ofphases, providing an output voltage from a frequency to voltage circuitto the second input, during the second phase providing a voltage from anintegration capacitor to a first input of a second amplifier, whereinthe output of the second amplifier is attached to the second input ofthe second amplifier and the output of the second amplifier is appliedto the output of the first amplifier via a third switch to charge aparasitic capacitance at the output of the first amplifier; during thethird phase providing the voltage of the output of the first amplifiervia a fourth switch to a voltage controlled oscillator circuit forcontrolling a frequency of an oscillation signal outputted by thevoltage controlled oscillator circuit, wherein during the second phaseand the third phase, the sampling capacitor is coupled to the secondinput of the first amplifier, the first switch is open, and the voltageof the output of the first amplifier is not provided to the samplingcapacitor; wherein the oscillation signal is provided to the frequencyto voltage circuit and the output voltage of the frequency to voltagecircuit is dependent on the frequency of the oscillation signal.

Various embodiments are described, further including producing a firstphase signal, a second phase signal, and a third phase signal by thevoltage controlled oscillator circuit, wherein the first phase signalcontrols the first switch and the second switch, the second phase signalcontrols the third switch, and the third phase signal controls thefourth switch.

Further various embodiments relate to a method for operating a freerunning oscillator system, the method including operating the oscillatorsystem during a plurality of phases of a comparison circuit of theoscillator system occurring periodically, including: during a firstphase of the plurality of phases, providing a reference voltage to afirst input of an amplifier via a first switch, wherein a second inputof the first amplifier is coupled to a first output of the amplifier andis coupled to a sampling capacitor, wherein a voltage of the firstoutput of the amplifier is provided to the sampling capacitor; during asecond phase of the plurality of phases, providing an output voltagefrom a frequency to voltage circuit to the first input via the firstswitch, providing the voltage of a second output of the amplifier to avoltage controlled oscillator circuit for controlling a frequency of anoscillation signal outputted by the voltage controlled oscillatorcircuit, wherein the amplifier produces an output at the first outputduring a first phase, and wherein the amplifier produces an output atthe second output during a second phase.

BRIEF DESCRIPTION OF THE DRAWINGS

In order to better understand various exemplary embodiments, referenceis made to the accompanying drawings, wherein:

FIG. 1 illustrates the prior art solution;

FIG. 2 illustrates that a parasitic capacitance is found at the outputof the amplifier;

FIG. 3 illustrates an FRO with a pre-charging amplifier to pre-chargethe output of the amplifier 121 to Vint;

FIG. 4 illustrates the signals used to control the switchers in thecomparator and how the comparator works;

FIG. 5 illustrates a close up plot of the Voo when Phc is high;

FIG. 6 illustrates another embodiment of an comparator that compensatesfor parasitic capacitors at the output of the amplifier; and

FIG. 7 illustrates a circuit implementation of the amplifier.

To facilitate understanding, identical reference numerals have been usedto designate elements having substantially the same or similar structureand/or substantially the same or similar function.

DETAILED DESCRIPTION

The description and drawings illustrate the principles of the invention.It will thus be appreciated that those skilled in the art will be ableto devise various arrangements that, although not explicitly describedor shown herein, embody the principles of the invention and are includedwithin its scope. Furthermore, all examples recited herein areprincipally intended expressly to be for pedagogical purposes to aid thereader in understanding the principles of the invention and the conceptscontributed by the inventor(s) to furthering the art and are to beconstrued as being without limitation to such specifically recitedexamples and conditions. Additionally, the term, “or,” as used herein,refers to a non-exclusive or (i.e., and/or), unless otherwise indicated(e.g., “or else” or “or in the alternative”). Also, the variousembodiments described herein are not necessarily mutually exclusive, assome embodiments can be combined with one or more other embodiments toform new embodiments.

Silicon-based FROs combine low phase noise with high accuracy. The highaccuracy is achieved by a switched-cap feedback loop with novelauto-zeroing solution for offset compensation whereas the low phasenoise is achieved using a voltage-controlled oscillator. The FRO has abest-in-class power supply rejection and can also operate at low supplyvoltages.

With the new embodiments described herein, the frequency accuracy of theprior art feedback FRO solution is even further improved. Theembodiments described herein are a further improvement of prior artsolution found in U.S. Pat. No. 10,250,269, which is hereby incorporatedby reference for all purposes as if fully set forth herein. Theembodiments described herein include the following features: low phasenoise (high figure of merit); high frequency stability over process andvoltage; high accuracy; low Allan deviation; high power supplyrejection; linear tuning range; and small area. The FRO solutionillustrated by the embodiments described herein will decrease the costof current products by: reduced trim costs; single trim at anytemperature; possible replacement of X-tal; reducing bill of materials;smaller form factor; and increased reliability by eliminating a(mechanically sensitive) X-tal.

FIG. 1 illustrates the prior art solution as found in U.S. Pat. No.10,250,269. The prior art FRO 100 is a ring oscillator with aswitched-cap based feedback loop. The offset of the amplifier is removedby means of auto-zeroing. The auto-zeroing solution removes the offsetand flicker noise and increases the effective gain which results in anincreased accuracy and higher frequency stability. The FRO 100 includesa switched capacitor based frequency to voltage converter (F2V) 110, acomparator 120, and the voltage controlled oscillator (VCO) 130. The VCO130 outputs two frequencies that are fed back to the F2V 110 which areused to control two switches 113 and 114. A switch capacitor 112 is usedwith a variable resistor 111 to control the output of the F2V 110. Theswitch capacitor 112 looks like a resistor based upon the frequency ofthe signal across it. The switch capacitor 112 and the variable resistor111 act as a voltage divider to produce a feedback voltage Vfb.

The comparator 120 may act as an integrator. The comparator 120 includesan amplifier 121, switches 123 and 124, an integration capacitor 122,and a sampling capacitor 125. The amplifier 121 may be an operationaltransconductance amplifier (OTA) or other type of appropriate amplifier.The amplifier 121 converts a voltage difference at the inputs to anoutput current. When the FRO is at the correct frequency, the output ofthe amplifier 121 is zero current. The switches 123 and 124 are switchedso that a reference voltage Vref is applied to the input of theamplifier 120 which charges the sampling capacitor 125 until it reachesa voltage of Vref. Then the switches 123 and 124 are switched so thatVfb is input to the amplifier 121, and the output current of theamplifier 121 will indicate any difference between Vfb and Vref. Thisoutput is integrated on Cint 122. This will cause the input to the VCO130 to adjust until the control loop settles.

FIG. 2 illustrates that a parasitic capacitance 126 is found at theoutput of the amplifier 121. When Vref is not equal to Vint, then theeffective gain is reduced due to the parasitic capacitance on Voo. Theeffective gain is reduced due to the fact that every period charge isneeded to change the voltage Voo of the output of the amplifier 121 fromVref to Vint. First, Cpar 126 is quickly charged by Cint 122 due to thecharge redistribution between Cint 122 and Cpar 126, and the remainingvoltage reduction is removed by the loop introducing an offset at theamplifier input in steady state (Vfb=Vref+Voffset). The offset reducesthe frequency accuracy because Vref is not equal to Vfb and thereforenot only dependent on R and C anymore.

The frequency inaccuracy due to Cpar 126 may be minimized by making thedifference between the values Vref and Vint as small as possible. Thedifference however is process, voltage, and temperature (PVT) dependent.The embodiments described herein provide a solution to remove thefrequency inaccuracy due to Cpar 126 in a robust way over PVT and toimprove the auto-zeroing solution.

FIG. 3 illustrates an FRO with a pre-charging amplifier to pre-chargethe output of the amplifier 121 to Vint. The FRO 300 uses the same F2V110 and VCO 130 as the FRO 100 in FIG. 1 . The Integrator 320 is similarto the comparator 120 in FIG. 1 , with the addition of the pre-chargeamplifier 327 and switches 328 and 329. The pre-charge amplifier 327 maybe, for example, an OTA or any other appropriate type of amplifier. Thepre-charge amplifier 327 pre-charges the parasitic capacitance Cpar 126to a voltage of Vint. This means that there will be no voltage reductionof Vint due to the need to charge Cpar 126.

A divider/phase generator 131 has been added to the output of the VCO130. The phase generator 131 generates three control signals Pha, Phb,and Phc. These control signals are used to control the various switchesin the comparator 320. Specifically, Pha controls switches 123 and 324,Phb controls switch 329, and Phc controls switch 328.

FIG. 4 illustrates the signals used to control the switchers in thecomparator and how the comparator works. FIG. 4 shows a plot of Voo withpre-charge 405 and Voo without pre-charge 410. Also plots of thecontrols signals Pha 415, Phb 420, and Phc 425 are shown. Pha controlsswitches 123 and 324. When Pha 415 is high Vref is sampled and stored onthe sampling capacitor 125. When Pha 415 is low Vfb is input into theamplifier 121. When Pha 415 goes low, then Phb 420 goes high to closeswitch 329. This allows the pre-charge amplifier 327 to charge theparasitic capacitor 126 to be equal to Vint. Once Phb 420 goes low, Phc425 goes high to close switch 328. This allows the output of theamplifier 121 to be connected to the VCO 130. As the input to theamplifier 121 is Vfb and the sampled Vref, the output of amplifier 121is based upon the difference in the input voltages that will lead tocorrection of the frequency of the VCO 130. FIG. 5 illustrates a closeup plot of the Voo when Phc 425 is high. As can be seen, the Voo withpre-charge 405 settles quickly to a constant value. The Voo withoutpre-charge 410 continues to rise during this period.

During operation without pre-charging, Cpar 126 is quickly charged byCint 122 when Vfb is applied to the amplifier 121. The voltage over Cint122 however is reduced due to the charge redistribution between Cint 122and Cpar 126. The voltage reduction is removed by the loop introducingan offset at the amplifier which effects a reduction of the effectivegain.

With pre-charging, Cpar 126 is first pre-charged with the pre-chargeamplifier 121 to the same voltage as Vint when the control signal Phb420 goes high and closes switch 329. So, there will be no voltagereduction of Vint due to Cpar 126, hence no offset is introduced at theamplifier 121.

FIG. 6 illustrates another embodiment of a comparator that compensatesfor parasitic capacitors at the output of the amplifier. The comparator600 includes an amplifier 621, switch 623, integrating capacitor Cint622, and sampling capacitor 625. The amplifier 621 includes two separateoutputs 628 and 629 and an internal switch that switches between the twooutputs. Further, each of the outputs 628 and 629 have their ownparasitic capacitance Cpar1 626 and Cpar2 627. The first output 628 isconnected to the output of the comparator 600 and Cint 622. The secondoutput 629 is connected to the second input of the amplifier 621 and thesampling capacitor 625. As a result the parasitic capacitances areindependent from one another and only effect their respective outputs.As a result the value of Vint is stored on both Cint 622 and Cpar2 627.This is accomplished by switching between the two outputs 628 and 629 ofthe amplifier 621. In this way the value of both Vref′ and Vint arepreserved during switching. Accordingly, the offset, due to Cpar2 isremoved, and therefore the FRO is only dependent on R and C. So, theeffective gain of the amplifier is reestablished again.

FIG. 7 illustrates a circuit implementation of the amplifier 621. Theimplementation is a standard implementation of the amplifier with twoseparate output stages 705 and 710. Switches 715 and 720 switch betweenthe two output stages 705 and 710. Parasitic capacitances 727 and 726are shown as being present at the outputs.

It should be appreciated by those skilled in the art that any blockdiagrams herein represent conceptual views of illustrative circuitryembodying the principles of the invention.

Although the various exemplary embodiments have been described in detailwith particular reference to certain exemplary aspects thereof, itshould be understood that the invention is capable of other embodimentsand its details are capable of modifications in various obviousrespects. As is readily apparent to those skilled in the art, variationsand modifications can be affected while remaining within the spirit andscope of the invention. Accordingly, the foregoing disclosure,description, and figures are for illustrative purposes only and do notin any way limit the invention, which is defined only by the claims.

What is claimed is:
 1. A free running oscillator, comprising: avoltage-controlled oscillator circuit comprising: an input configured toreceive an input voltage; and an output configured to provide anoscillation signal, wherein the input voltage controls a frequency ofthe oscillation signal; a frequency-to-voltage circuit comprising: aninput configured to receive the oscillation signal; and an outputconfigured to produce a voltage dependent on a frequency of theoscillation signal; a comparison circuit comprising: an amplifierincluding a first input, a second input, a first output, a secondoutput, and a first switch, wherein the first output of the amplifierand the second output of the amplifier are each determined based upon adifference in voltage between the first input of the amplifier and thesecond input of the amplifier, the first switch selectively activatesone of the first output of the amplifier and the second output of theamplifier, and the second output of the amplifier is connected to thesecond input of the amplifier; a second switch with an output connectedto the first input of the amplifier, a first input connected to areference voltage, and a second input connected to the output offrequency-to-voltage circuit, wherein the second switch selectivelyswitches between the first input of the second switch and the secondinput of the second switch to produce an output signal at the output ofthe second switch; a sampling capacitor connected between the secondinput of the amplifier and a ground; and an integration capacitorconnected between an output of the comparison circuit and the ground. 2.The free running oscillator of claim 1, wherein the voltage-controlledoscillator circuit produces at least one control signal configured tocontrol the first switch and the second switch.
 3. The free runningoscillator of claim 2, wherein the voltage-controlled oscillator causesthe free running oscillator to operate in a selected one of a samplingphase and a comparison phase based on the at least one control signal.4. The free running oscillator of claim 3, wherein, in the samplingphase, the at least one control signal causes the comparison circuit tosample the reference voltage and store the sampled reference voltage onthe sampling capacitor.
 5. The free running oscillator of claim 4,wherein, in the comparison phase, the at least one control signal causesthe comparison circuit to compare a voltage on the sampling capacitorwith the output of the frequency-to-voltage circuit.
 6. The free runningoscillator of claim 1, wherein the amplifier is an operational amplifierconfigured to produce an amplifier output signal having a current thatis based upon the difference in voltage between the first input of theamplifier and the second input of the amplifier.
 7. A method foroperating a free running oscillator system, wherein a comparison circuitof the oscillator system is configured to operate in a plurality ofphases that occur periodically, comprising: during a first phase of theplurality of phases, providing a reference voltage to a first input ofan amplifier via a first switch, wherein a second input of the amplifieris coupled to a first output of the amplifier and is coupled to asampling capacitor, wherein a voltage of the first output of theamplifier is provided to the sampling capacitor; during a second phaseof the plurality of phases, providing an output voltage from afrequency-to-voltage circuit to the first input via the first switch,providing the voltage of a second output of the amplifier to avoltage-controlled oscillator circuit for controlling a frequency of anoscillation signal outputted by the voltage-controlled oscillatorcircuit, wherein the amplifier produces a first output signal at thefirst output during a first phase, and wherein the amplifier produces asecond output signal at the second output during a second phase.
 8. Themethod of claim 7, wherein the amplifier is an operational amplifierconfigured to produce an amplifier output signal having a current thatis based upon the difference in voltage between the first input andsecond input of the amplifier.
 9. The method of claim 7, furthercomprising: during the first phase, sampling the reference voltage; andstoring the sampled reference voltage on the sampling capacitor.
 10. Themethod of claim 9, further comprising: during the second phase,comparing the sampled reference voltage stored on the sampling capacitorwith the output of the frequency-to-voltage circuit.
 11. The method ofclaim 10, further comprising: during the second phase, integrating theresult of the comparison between the stored reference voltage and theoutput of the frequency-to-voltage circuit using an integrationcapacitor.
 12. The method of claim 11, wherein the integration capacitoris coupled between an output of a comparison circuit that includes theamplifier and a ground.
 13. The method of claim 7, further comprising:with a voltage-controlled oscillator circuit, producing at least onecontrol signal configured to control at least the first switch.
 14. Themethod of claim 11, further comprising: controlling at least a secondswitch to selectively switch between the first phase and the secondphase, wherein the second switch is configured to selectively activateone of the first output of the amplifier and the second output of theamplifier.